We can modify gradable adjectives with adverbs to say that something has more or less of a quality.
More of a quality: very, rather, really, extremely
Less of a quality: a bit, a little, fairly, slightly
Gradable adjectives: interesting, scared, surprised, difficult, tired, big, busyeg. I saw a really interesting movie last night.
That restaurant is fairly busy.
We can’t use the same modifying adverbs with non-gradable adjectives. We must use intensifying adverbs to emphasise their quality.
Intensifying adverbs: absolutely, completely, entirely, totally, utterly
Non-gradable adjectives: fascinating, terrified, amazed, impossible, shattered, awful, excellent, huge, superb, terrible
eg. I was completely shattered by Friday.
The concert was absolutely superb.
NB: We can use ‘really’ for both gradable and non-gradable adjectives.